The invention relates to an extruder for processing and producing rubber and thermoplastic plastics materials.
In the field of extruder technology, it has long been known to construct an extruder so that metal pins protrude through its housing into the processing chamber for processing the extruded material in mixing sections. In addition, the flights of the extruder screw are interrupted at the locations where the pins protrude into the extruder housing. Such extruders are known, for example, from German Offenlegungsschrift No. 2 235 784 or German Offenlegungsschrift No. 3 003 615.
These extruders are distinguished by their very good mixing and homogenising effect upon the material to be processed and permit also an increased throughput of material through the extruder per unit of time whilst the r.p.m. of the screw remains constant. These advantages have resulted in pin-barrel extruders becoming the most commonly used mixing and homogenising extruders in the last 15 years (not least as a consequence of constant improvement).
Independently of this, a mixing section for an extruder has been developed, which has become known as a transfer mix mixing section (DE-A 11 42 839). This mixing section is substantially characterised in that both the extruder screw and the internal wall of the extruder housing are provided with grooves and flights over a predetermined length, the thread depth of the extruder screw, when viewed in the longitudinal direction of the extruder housing, decreasing to zero and subsequently increasing again at the same rate as the thread depth of the grooves in the housing respectively increases and decreases again. As a result of this configuration for the extruder screw and housing, the extruded material can be conveyed from the screw grooves into the housing grooves, thereby having a good mixing effect upon the extruded material, and such conveyance is complete when the mixing section rotates.
Compared with the pin-barrel extruder, the transfer mix extruder could claim for itself a certain corner of the market, especially when the overall length of the extruder had to be kept small. However, the comparatively high cost of manufacture of the mixing section is a disadvantage of such a construction.